Stable n‑Type Conduction in WO x ‑CNT Hybrid Films

Abstract

Nanostructured hybrid films composed of tungsten oxide (WO _x ) nanoclusters and vertically aligned carbon nanotubes (CNTs) were synthesized through a combination of chemical vapor deposition and supersonic cluster beam deposition. The use of a cluster source enabled the direct fabrication of oxygen-deficient, nonstoichiometric WO _x nanoclusters, which decorated the CNT sidewalls with a characteristic "beaded necklace-style" morphology. Electrical resistance measurements under ethanol exposure in ultrahigh vacuum revealed a distinct behavior consistent with n-type conduction, unlike the intrinsic p-type behavior of pristine CNTs and of WO _x films. This inversion is linked to the appearance of an interfacial charge transfer from the oxygen vacancies in the defective WO _x nanoclusters to the CNTs, which injects electrons into the CNT network and shifting its Fermi level, thereby inverting the conduction type. Notably, this n-type conduction response remained stable even after prolonged air exposure. These results propose a viable approach to achieving air-stable n-type doping in CNT-based nanostructures.

Keywords: carbon nanotube hybrids; gas sensing; hybrid nanostructures; n-type conduction; supersonic cluster beam deposition; tungsten oxide nanoclusters.

1

SEM images acquired on the various samples. (a) CNTs forest; (b) NS-WO _x /CNT hybrid deposited by the supersonic cluster beam source; (c) 200 nm NS-WO _x layer on a Si wafer capped with native oxide; and (d) 25 nm WO _x layer deposited on a CNT forest by magnetron sputtering.

2

TEM characterization of the nanoparticle aggregates forming the NS-WO _x layer on a Si wafer. The left panel shows the selected-area electron diffraction (SAED) pattern acquired from the sample, indicating partial crystallinity. The central and right panels show TEM micrographs from different regions of the sample. The inset in the right panel presents a magnified view of the area highlighted in red, along with the corresponding fast Fourier transform (FFT) pattern.

3

(a) W 4f and (b) O 1s spectra acquired on the different samples, as indicated in each panel. The filled colored dots represent the experimental data; the continuous gray line shows the best fit to the data using the spectral components (filled line profiles) and the background (dashed line) reported in each spectrum.

4

Sensitivity measurements of the various materials upon exposure to EtOH in UHV (partial pressure p = 2 × 10^–5 mbar). The dashed line on the right indicates when EtOH exposure was stopped. The data for the WO _x film grown by DCMS on CNTs (yellow) were multiplied by 5 to show that they follow the same trend as the pristine CNT sample but with reduced intensity. Shaded regions represent the corresponding error margins.